Become a Control Freak with K and Reap the Rewards – Part 2

30 June 2016

In last week's instalment, we looked at how potassium deficiency can limit yield, crop quality and resilience. In this segment, I will identify the strategies to identify a K deficiency from the moment it begins. In doing so, you will minimise problems through developing much better control of potassium.

Five Ways to Identify a Damaging K Deficit

1) The visible evidence of a K shortage begins with burnt leaf tips. This deficit may be related to a lack of potassium in the soil, the antagonism of potassium uptake by other minerals, or a shortage of moisture. Potassium base saturation should ideally be a minimum 3% for pastures, sugar cane and cereals, but it is required at higher levels for more bushy plants and fruit or vine crops. Here, there should be a minimum 5% of potassium ions attached to the clay colloids in your soil. The other three key cations (calcium, magnesium and sodium) can all limit the uptake of potassium if they are oversupplied in the soil. However, the mineral most likely to stomp down on potassium availability is nitrogen. If nitrogen is oversupplied, the first mineral impacted is potassium. The other K-limiting condition is drought or dry spells. Potassium is the second most abundant mineral in the soil and it is only supplied by soil solution. When there is no moisture around the roots, there is no delivery of potassium to the plant. The stems quickly weaken and the plant droops.

During my recent visit to Wales I encountered this wheat crop that had suddenly descended into potassium deficiency (burnt leaf ends) due to an extended dry period.

2) A Plant Sap Potassium Meter may be the most valuable of all monitoring tools. You can use it to determine if you have enough potassium in the plant, but it can also be used in a different manner to manage the "money mineral" more efficiently. Here's how it works: the super-mobile potassium mineral abandons the lower leaves the moment there is insufficient K to go around. It heads to where it is most needed, but the departure is sorely felt. The sap pH in the lower leaves drops in the absence of this alkalising mineral and the ensuing acidity creates the preferred environment for invasive pathogens. You may have wondered why disease so often begins in the lower leaves and now you have an answer. This simple monitoring strategy involves measuring the potassium level in the first fully developed leaf. This is the testing site for all in-field crop monitoring. Sap is extracted from this leaf with a garlic crusher and dripped into the sink of the K meter. The digital meter delivers a reading and this is the first half of the two-part protocol. Now you repeat the process using the bottom leaves of the plant. There should be little difference between both testing sites. If the potassium levels in the bottom leaves of the plant are more than 10% lower than the upper leaves, then you have identified a potassium deficiency and immediate action is essential.

3) There is a cheaper option if you can not afford a potassium meter and this tool also offers other guidance. The Plant Sap pH Meter can be used to test the top leaves and the bottom leaves, just like a potassium meter. When the sap pH is lower in the bottom leaves, it is usually related to a potassium deficiency. Sap pH in the first fully developed leaf is also a prime indicator of other likely problems. It has been determined that a pH of 6.4 is the ideal for most crops and, if you can keep your crop in this zone, there will be a big smile on your face and your bank manager will also be beaming. A sap pH below 6.4 heralds the lack of a key alkalising mineral (usually calcium, magnesium or potassium), and an acidic plant has been found to encourage fungal disease. Conversely, if the sap pH is above 6.4, then there may be too little sulfur or phosphorus in the leaf. In this instance, there will be more likelihood of insect pressure.

4) The NTS Water Stress Meter can also be used as a guideline for potassium action. This inexpensive, infrared temperature gun is an invaluable, user-friendly tool, which provides instant feedback as to the hydration of your crop. The gun is pointed at a white sheet of paper positioned 30 cm from the gun and the trigger is pulled. A beam of infrared light then measures the air temperature between the barrel of the gun and the paper. This provides a measure of ambient air temperature. Now you point the beam at the leaf of your crop, from the same distance (30 cm). You are now measuring the temperature of the leaf surface. Plants practice something called evaporative cooling, where the surface of the leaf should always be maintained at lower levels than the ambient air temperature. This is an essential plant mechanism for maintenance of many metabolic processes. If the leaf is the same, or a higher temperature than ambient air temperature, that plant is crying out for water. This can be an immensely important monitoring tool if you have irrigation and can respond to this cry for help. However, it is not usually thought to be of value in dryland farming. This is not necessarily the case. It is a safe bet to assume that if the plant is moisture stressed, then shortly afterwards, there will be limited potassium availability. In dryland farming, this understanding can provide the guidelines for foliar potassium applications, which can ensure maximum yield potential even in the face of adversity.

5) Finally, there is another incredibly cheap option to monitor sap pH from upper and lower leaves, and potential potassium deficiency. This involves purchasing a set of NTS pH Test Strips for $4.95 (AUD). These are a little different from the common pH strips used to monitor swimming pools, because they measure in finer gradients (i.e., increments of 0.1). Here, you simply squeeze sap from the top and bottom leaves with a garlic crusher and then separately drip this sap onto the two pH strips. The colour change on the strips will indicate relative pH. If the bottom leaves have a lower pH than the top leaves (according to the chart supplied), then you have, in all likelihood, identified a potassium deficiency and it is time to act.

How to Act

What is the best solution to address the K shortage you have identified? This depends upon what else is lacking, but commonly it is a good idea to consider a foliar application of potassium sulfate. This substance also delivers plant-available sulfur to boost protein levels in cereal and legume crops, so it is a double whammy. You can only dissolve 10 kg of potassium sulfate in 100 L of water, as it is less soluble than many fertilisers. However, 100 L per hectare is a good water rate for field crops. NTS Fulvic Acid Powder™ should always be combined with the potassium sulfate foliar, at a rate of 120 – 250 g per hectare, to magnify the uptake of sulfur and potassium while also providing other plant benefits associated with fulvic acid.

If you are also lacking phosphorus, then you might consider an application of MKP (also buffered and magnified by fulvic acid). The appropriate rate here is just a couple of kg per hectare of MKP with 250 g of the NTS Fulvic Acid Powder™.

Finally, you have the option of K-Rich™, the super-concentrated foliar product from NTS. This fully-buffered, gentle form of potassium delivers 35% potassium rapidly into the leaf and it has no unwanted tag-ons.

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